1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a semiconductor device having a load short circuit protection function.
2. Description of the Related Art
With the current advances in technology, a semiconductor device having an over-current protection function for protecting a semiconductor device from over-current that flows when load is shorted is in practical use. This over-current protection function is for suppressing the power consumption of the semiconductor device and protecting the semiconductor device from breakdown by controlling the current that flows through the semiconductor device when such an abnormality as a load short circuit occurs. FIG. 10 shows a circuit configuration of a conventional semiconductor device disclosed in U.S. Patent Gazette No. 4553084. When the gate voltage for driving the output MOSFET 101 is applied to the control terminal 102, this gate voltage is applied to the gates of the output MOSFET 101 and detection MOSFET 104 for monitoring the output current via the gate register 103, main current flows through the output MOSFET 101, and detection current flows through the detection MOSFET 104. This detection current is set to a value that is about 1/1000-1/10000 of the main current. If the circuits connected to the output MOSFET 101 are in normal status, the detection voltage based on the detection of the detection resistor 105 is kept smaller than the threshold voltage of the over-current protection MOSFET 106, and MOSFET 106 is kept in OFF status. FIG. 11 shows the drain current ID, which is a main current with respect to the voltage VDS between the source and drain of the output MOSFET 101 in this status.
If such an accident as a load short circuit occurs to a load or to a load circuit 116 connected to the output MOSFET 101 and excessive main current flows into the output MOSFET 101, the detection current that flows through the detection MOSFET 104 increases, and detection voltage, based on the drop in voltage of the detection resistor 105, also increases. And if the detection voltage exceeds the threshold voltage of the over-current protection MOSFET 106, the MOSFET 106 turns ON and the input to the output MOSFET 101 is divided by the gate resistor 103 and the over-current protection MOSFET 106, so the potential at the potential point A dramatically drops. If the potential at the potential point A drops, the main current of the output MOSFET 101 and the detection current of the detection MOSFET 104 also decrease as the gate voltage drops, and a breakdown of the output MOSFET 101 by the over-current is prevented. In this case, the potential of the potential point A drops according to the magnitude of the detection voltage to be applied to the gate of the over-current protection MOSFET 106.
Also recently a circuit for detecting the voltage between the source and drain of the output MOSFET and a circuit for adjusting the voltage between the gate and source of the output MOSFET and changing to a desired current limit value are proposed. FIG. 12 shows the current limiting characteristics of a conventional semiconductor device stated in “Smart Highside Power Switch”, Data sheet BTS 6143D, p. 13, FIG. 3a, Infineon Technologies AG in Germany, Oct. 1, 2003, which is disclosed on internet and found by searching on Feb. 17, 2004 at URL: http://www.infineon.com/cmc_upload/documents/014/444/BTS6143D —20030925.pdf. In the case of the technique disclosed in “Smart Highside Power Switch”, five sets of over-current detection circuits are connected in parallel so as to implement five stages of current limiting, as shown in FIG. 12. By this, power consumption can be suppressed more closely by changing the current limit value for each voltage of the voltages between the drain and source of the output MOSFET.
In this case, a double diffusion type field effect transistor composed of cells, shown in FIG. 13 and FIG. 14, is normally used for the output MOSFET 101 and the detection MOSFET 104 shown in FIG. 10. A MOSFET where channels are formed in the vertical direction with respect to the semiconductor substrate face, such as the case of the double diffusion type field effect transistor, is called a “vertical MOSFET”. FIG. 13 is a plain view of a vertical MOSFET and FIG. 14 is a cross-sectional view along the XIV-XIV line in FIG. 13. This vertical MOSFET has a structure where a plurality of unit cells, in which one source electrode 107 is surrounded by the gate electrode 108, and an epitaxial layer 110 having low density n-type impurities (n−) is formed on a semiconductor substrate 109 having high density n-type impurities (n+), and using these as a drain area, a double diffusion area which is composed of a base area 111 having p-type impurities and a source area 112 having high density n-type impurities is formed in the epitaxial layer 110, and the gate electrode 108, source electrode 107 and drain electrode 113 are formed respectively, as shown in FIG. 14.
This double diffusion type field effect transistor becomes more and more miniaturized every year so as to decrease the ON resistance per unit area. For example, the conventional semiconductor device shown in FIG. 15 and FIG. 16 is known as a structure with less ON resistance. FIG. 15 is a plain view of a conventional semiconductor device, and FIG. 16 is a cross-sectional view along the XVI-XVI line in FIG. 15. This conventional semiconductor device is a semiconductor device with a trench structure where the gate electrode 108 is buried in the base area 111 and epitaxial layer 110.
A merit of using a double diffusion type field effect transistor of which the ON resistance per unit area is low is that the size of the output MOSFET can be decreased. However if a double diffusion type field effect transistor of which the ON resistance per unit area is low is used, the conventional semiconductor device shown in FIG. 10 has the following problem. That is, unless the current limit value is decreased for the amount of the decreased size of the output MOSFET, the heating value when such an abnormality as a load short circuit occurs increases, which makes it easier for the semiconductor device to breakdown, and if the current limit value is decreased, the semiconductor device cannot be used for high output applications where large current flows.
In the case of the technique disclosed in “Smart Highside Power Switch”, the current limit value is increased in an area where voltage is low, that is a safe operation area, and the current limit value is decreased in an area where voltage is high, that is outside the safe operation area. The structure of the technique disclosed in “Smart Highside Power Switch” requires a voltage detection circuit and a circuit to change the current limit value for each stage, so the circuit scale is about five times larger than the semiconductor device in FIG. 10, which is the downside of this approach.